Iapetus

Iapetus from Cassini, 2004. This
mosaic image was obtained with the spacecraft's narrow angle camera
on Dec. 31, 2004, at a distance of about 172,400 km (107,124 miles)
from the moon and at a Sun-Iapetus-spacecraft, or phase, angle of
50 degrees. The image scale is 1 km (0.6 miles) per pixel. The image
has been contrast enhanced to aid visibility of surface features.

discovery

1671, by Giovanni Cassini

semimajor axis

3,560,820 km (2,213,060 miles)

diameter

1494 × 1498 × 1425 km
(929 × 931 × 886 miles)

mean density

1.27 g/cm3

escape velocity

0.61 km/s (2196 km/h, 1365 mph)

orbital period

79.33 days (79 days 8 hours)

orbital eccentricity

0.029

orbital inclination

15.47°

axial period

synchronous

visual albedo

0.04 to 0.5

Iapetus from Voyager 2, on Aug. 22,
1981

Latest news (July 2012) >>> Images from the Cassini
spacecraft of Saturn's moon Iapetus have shed light on the possible
origin of huge landslides seen on the moon's surface. These landslides,
of a type known as sturzstroms or "long-runout landslides",
travel amazing distances horizontally – up to 30 times further
than the material falls vertically. According to an article published
in Nature Geoscience, in July 2012, localized heating of
ice rubble could be a crucial factor in explaining how this happens.[1] See the section "Giant landslides" below for more details.

Iapetus is an outer moon of Saturn, also
known as Saturn VII, that was discovered by Giovanni Cassini in 1671. It is remarkable in that its brightness changes by a factor of seven as it moves around its orbit – a feature that has led
to it being called the "yin and yang" moon. Whereas the leading hemisphere
of Iapetus is as dark as asphalt with a slight reddish color, the trailing
hemisphere is very bright.

The great ridge of Iapetus

In December 2004, the Cassini spacecraft
discovered a further extraordinary feature of Iapetus: a topographic ridge
that coincides almost exactly with the geographic equator. The ridge is
conspicuous in the top image on this page as an approximately 20-km wide
(12 miles) band that extends from the western (left) side of the disk almost
to the day/night boundary on the right. On the left horizon, the peak of
the ridge reaches at least 13 km (8 miles) above the surrounding terrain.
Along the roughly 1,300 km (800 mile) length over which it can be traced
in this image, it remains almost exactly parallel to the equator within
a couple of degrees. The physical origin of the ridge has yet to be explained.
It is not yet clear whether the ridge is a mountain belt that has folded
upward, or an extensional crack in the surface through which material from
inside Iapetus erupted onto the surface and accumulated locally, forming
the ridge.

Giant landslides

Iapetus is also the site of unusual landslides, seen only rarely elsewhere
in the solar system, on Earth and Mars. These landslides, called sturzstroms or "long-runout landslides", involve debris traveling up to 30 times further
horizontally than it does vertically. Their origin is still not known for
sure, but, according to an analysis of Cassini observations of sturzstroms
on Iapetus by a team of planetary geologists, published in 2012, flash heating
of icy rubble is likely a key ingredient.[1] When such a landslide
begins it heats the contact points between fragments of ice debris, melting
them and leading to a very low coefficient of friction; this enables the
falling material to travel out very far from the initial site of the fall,
as if it is running on virtually frictionless rollers.

Light and dark regions

The scene in the top image is dominated by the dark, heavily-cratered region,
called Cassini Regio, that covers nearly an entire hemisphere of Iapetus.
The view is centered on the moon's equator and on roughly 90° west longitude
– a location that always faces the direction of Iapetus's orbital
motion around Saturn. Within Cassini Regio, and especially near the equator,
dark deposits with a visual reflectivity of only about 4 percent coat nearly
everything with remarkable uniformity. However, at latitudes of about 40°,
the surface transitions to a much brighter, icy terrain near the pole where
the brightest icy materials have reflectivity over 60 percent. However,
this region is not uniform: Close inspection reveals that the surface is
stained by crudely north-south trending wispy streaks of darker material,
typically a few kilometers wide and sometimes tens of kilometers long. An
ancient, 400-km wide (250 miles) impact basin appears just above the center
of the disk. The basin is heavily overprinted by more recent, smaller impact
craters. The basin rim is delineated by steep scarps that descend to the
basin floor. Many of these scarps, as well as walls of nearby craters, appear
bright, probably due to exposed outcrops of relatively clean ice. Particularly
at mid-latitudes, the brightest scarp exposures appear to face away from
the equator (i.e. toward the pole). Often, the opposite south-facing scarps
are stained with the lower-brightness material.

The origin of Cassini Regio is a long-standing debate among scientists.
According to one theory, the dark material may have erupted onto Iapetus's
icy surface from the interior. Another theory holds that the dark coating
consists of debris that was ejected by impact events on dark, outer satellites
of Saturn, such as Phoebe, and then swept
up by Iapetus as it moved in its orbit. Details of this Cassini image mosaic
do not definitively rule out either of the theories. However, they do provide
important new insights and constraints.

The uniform appearance of the dark materials at the equator, the apparent
thinning and spottiness of the dark materials at progressively higher latitudes
and dark wispy streaks near the distal margin of Cassini Regio strongly
suggest that dark material was emplaced as a coating. One of the important
new results is that no clear evidence can be found that erupted fluids have
resurfaced Cassini Regio. The high density of impact craters argues that
the terrain underlying the dark coating is relatively ancient and has not
been eradicated by its emplacement. Thus, Cassini Regio may have had its
origin in plume-style eruptions in which dark particulate materials accumulated
on the surface as fallout, perhaps in conjunction with the creation of the
equatorial ridge. On the other hand, the dark deposits in Cassini Regio
may be a surface coating consistent with, and perhaps more simply explained
by, the fall of dark materials from outside.

This latter idea received a powerful boost in 2009 with the discovery of
a colossal, infrared-emitting ring around Saturn, the material of which
appears to have come from Phoebe. It now seems likely that the dark material
on the surface of Iapetus consists of debris swept up from this previously
unknown ring. For more, see rings of Saturn.

Far-out speculation

The extreme albedo range displayed by Iapetus had earlier prompted the suggestion
that the brightness variations might be artificial. For example, Donald
Goldsmith and Tobias Owen in The Search
for Life in the Universe2 (1980) wrote:

This unusual moon is the only object in the
Solar System which we might seriously regard as an alien signpost - a
natural object deliberately modified by an advanced civilization to attract
our attention ...

Cassini close flyby of 2007

On September 10, 2007, Cassini flew past Iapetus at a distance from the
surface of only 1,640 kilometers (1,000 miles) – almost 100 times
closer than the 2004 flyby. It sent back startling images, some of which
are shown below, that promise to revolutionize our knowledge of this strange
moon.

The great ridge on Iapetus seen in
profile

Mountainous terrain that reaches about 10 km (6 miles)
high along the unique equatorial ridge of Iapetus. Above the middle
of the image can be seen a place where an impact has exposed the bright
ice beneath the dark overlying material. The image was taken on Sept.
10, 2007, with Cassini's narrow-angle camera at a distance of approximately
3,870 km (2,400 miles) from Iapetus. Image scale is 23 meters (75
feet) per pixel.

Cassini close-up of the patchy, bright and dark mountains
originally identified in images from Voyager taken more than 25 years
earlier. The terrain seen here is located on the equator of Iapetus
at approximately 199° west longitude, in the transition region
between the moon's bright and dark hemispheres. North is up. The image
was taken on Sept. 10, 2007, with Cassini's narrow-angle camera at
a distance of approximately 9,240 km (5,740 miles) from Iapetus. Image
scale is 55 m (180 ft.) per pixel.

Dark material splatters the walls and floors of craters
in the surreal, frozen wastelands of Iapetus. This image shows terrain
in the transition region between the moon's dark leading hemisphere
and its bright trailing hemisphere. The image was taken on Sept. 10,
2007, with Cassini's narrow-angle camera at a distance of approximately
6,030 km (3,750 miles) from Iapetus. Image scale is 36 m (118 ft.)
per pixel.

Cassini surveys a bright landscape coated by dark
material on Iapetus. This image shows terrain in the transition region
between the moon's dark leading hemisphere and its bright trailing
hemisphere. The image was taken on Sept. 10, 2007, with Cassini's
narrow-angle camera at a distance of approximately 5,260 km (3,270
miles) from Iapetus. Image scale is 32 m (105 ft.) per pixel.